1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of a document.
2. Description of the Related Art
Conventionally, there have been image reading apparatuses having an automatic document feeder (hereinafter, referred to as ADF) used in image forming apparatuses such as digital copying machines. Such an image reading apparatus having the ADF moves and stops a reader unit provided below the ADF at a set position, that is, at a position approximately below a platen roller in the ADF, by a motor. Then, light is emitted from a lamp unit, the document is scanned while being conveyed between the platen roller and a glass plate, and the image is read by the by reader unit based on the reflected light.
As illustrated in FIG. 12, the document reader unit in the ADF of a conventional digital copying machine includes a platen roller 524 and a platen glass 516, which is a transparent document guide unit. A document P is conveyed in contact with the surface of the platen glass 516.
To ensure the conveyance properties of the document P, charging of the platen glass 516 is prevented by providing an earth spring (not illustrated) on the longitudinal direction (document width direction) end portion of the platen glass 516. However, there is a problem that dust, such as toner particles, paper dust, or rubber dust, produced by the document conveyance accumulates on the platen glass. In particular, if dust accumulates at the document reading position, a problem arises in that a streaky image is produced during image recording.
More specifically, as illustrated in FIG. 12, dust G conveyed with the document P slides between the document P and the platen glass 516. Thereafter, although the document P separates from the platen glass 516 and is conveyed along a conveyance guide 517, the dust G is electrostatically attracted to the platen glass 516, where it remains. At this stage, if the dust G is at a document reading position 99, the dust is continuously read by the reader unit during the reading operation of the document P. Consequently, image streaks are produced.
Japanese Patent Application Laid-Open No. 10-112771 discusses forming a thin film, which is transparent and conductive, on the glass surface of a contact image sensor.
Further, in the image reading apparatus (refer to FIG. 13), the platen glass 516 is coated with a conductive coating 516c on a document passage face of the platen glass 516. In addition, Japanese Patent Application Laid-Open No. 2005-184069 discusses preventing image streaks by sticking an aluminum sheet 516a extending in the width direction of the document, so that the static elimination effects are strengthened and the dust G is not electrostatically attracted to the platen glass 516.
According to the configuration discussed in Japanese Patent Application Laid-Open No. 2005-184069, dust G, such as paper dust, adhered to the document P is conveyed with the document P, and is slid between the platen glass 516 and the document P. The charge of the surface of the platen glass 516 is removed by the aluminum sheet 516a and a conductive layer (conductive coating) 516c on the surface of the platen glass 516. Consequently, the force of the electrostatic adsorption does not extend to among the document P, dust G, and platen glass 516, and the force due to friction becomes dominant.
Therefore, the frictional force between the document P and the dust G is stronger than the frictional force between the dust G and the platen glass 516, so that the dust G is conveyed with the document P. Consequently, the occurrence of image streaks due to the accumulation of the dust G on the platen glass 516 can be prevented.
Further, even if the dust G accumulates on the platen glass 516, as long as the dust G is not electrostatically attracted to the platen glass 516, the dust G is easily removed by the cleaning performed by the leading edge of the next document P, thereby preventing streaky images from being produced.
However, in recent years, there have been increasing demands placed on the ADF, such as faster reading speed (document conveyance speed), reduced size, and ability to handle various kinds of document passage.
For the demands concerning reducing the size of the apparatus, and especially for the passage of a thick document, in the configuration of the conventional apparatus illustrated in FIG. 13, the document is pressed against the platen glass with a strong abutting pressure as a result of a curved document-conveyance path and a very stiff thick document. This is because the document is conveyed in a direction intersecting the surface of the platen glass, from the upstream with respect to the platen glass in the conveyance direction.
Especially when thick paper of, for example, 200 g/cm2 or more is repeatedly passed, the conductive coating peels off from the glass due to the document repeatedly sliding over the platen glass face at the same position on the platen glass face. The following problems arise if the conductive coating on the platen glass face is abraded.
Due to the abrasion of the conductive coating, the above-described static elimination effects are impaired. If paper is repeatedly passed in a state where the static elimination effects are impaired, dust adheres to the platen glass. Consequently, streak image defects will occur.
Further, due to the abrasion of the conductive coating, the reading luminance fluctuates during image reading. This is because the light transmittance of the platen glass differs depending on whether the conductive coating is present or not. The fluctuation of the reading luminance has an adverse effect on the output image density.
This problem can be dealt with by applying a correction to the reading luminance if the conductive coating is uniformly present on the platen glass. However, if the conductive coating is locally abraded, there will be some portions of blank glass from which the conductive coating is abraded and some reading portions on which the conductive coating is not abraded.
Consequently, a difference in the reading luminance will be produced between the image of a document read via a blank glass portion and the image of a document read via a portion on which the conductive coating is present. Thus, non-uniform image luminance data will be obtained.
Accordingly, due to the abrasion of the conductive coating, a difference in image density can occur between the portions from which the conductive coating is abraded and is not abraded as a result of fluctuations in the light transmittance when the paper passes over the platen glass.